Evolution of Ageing - Disposable Soma Theory

Disposable Soma Theory

A third mainstream theory of ageing, the Disposable soma theory, proposed in 1977 by Thomas Kirkwood, presumes that the body must budget the amount of energy available to it. The body uses food energy for metabolism, for reproduction, and for repair and maintenance. With a finite supply of food, the body must compromise, and do none of these things quite as well as it would like. It is the compromise in allocating energy to the repair function that causes the body gradually to deteriorate with age.

The term disposable soma came from the analogy with disposable products—why spend money making something durable, if it will only be used for a limited amount of time?

The disposable soma theory has great appeal because its basis is so sensible and intuitive, but there are arguments against it. The theory clearly predicts that a shortage of food should make the compromise more severe all around; but in many experiments, ongoing since 1930, it has been demonstrated that animals live longer when fed substantially less than controls. This is the caloric restriction (CR) effect, and it cannot be easily reconciled with the Disposable Soma theory. Though by decreasing energy expenditure the damage generated (by free radicals for instance) is expected to be reduced and the total energy budget might indeed be reduced, but the investment in repair function might still be relatively the same. But dietary restriction has not been shown to increase lifetime reproductive success (fitness), because when food availability is lower reproductive output is also lower. So CR does thus not completely dismiss disposable theory.

Experimentally, some animals lose fertility when their life spans are extended by CR and some suffer no appreciable loss. Males, for example, typically remain fertile when underfed, while females do not. And, even females present an enigma because their fertility decline is not tightly coupled to their longevity gain. For example, in female mice that are restricted to 60% of a free-feeding diet, reproduction is shut down altogether. But female life span continues to increase linearly right up to the threshold of starvation - around 30% of free-feeding levels.

A difficulty with the disposable soma theory is that the energy required for maintenance and repair would appear to be relatively minor when compared to the energy required for gestation (repair should take less energy than producing an entire new organism). Yet gestating animals seem able to perform the maintenance while post-reproductive animals do not. A similar difficulty is that male animals seem to have similar life spans as females despite the apparently higher energy requirement for gestation and other reproductive activities.